You have discovered one of the fundamental differences between Java and C++;
In C++, when you allocate and then delete a block of memory, it is immediately
combined with adjacent free blocks, and the free memory chain can be scanned
quite quickly;
In your case, you simply dereference an array, which will not be garbage
collected until
1) Some free time is available (never true since you thread will have higher
priority as long as memory is available)
2) Your allocation request cannot be satisfied
Eventually, you are probably running garbage collection every couple of loops;
GC can be slow, and is very implementation dependent;
If you want the slower sequence at the start of your run, run your Java program
loop backwards, from NUM_MAX_BYTES to 1;
This is an inherent result of using Java's chosen memory model.
If you were only interested in reducing the variance, if you waited a short
will after each allocation (Thread.sleep(1)) you might find the variance
reduced, but Java would take a minimum of NUM_MAX_BYTES milliseconds to run, 5
minutes in your example. Also, I am not sure at all how Thread.sleep(n) works
on a platform where the clock resolution is low, as on most Windows platforms.
At 06:21 PM 8/30/00 +0800, you wrote:
>guys:
>
>i am doing a benchmark on java memory management. i wrote an app to allocate
>and free memory from 1 to 300k bytes. it runs forever. i notice it takes
>longer to allocate additional memory as time goes on. the same app only
>takes 9 seconds on C++. here's the code listing. can anyone pls help out?
>
>tks,
>peter
>
>=======JAVA CODE LISTING =======
>public class Class1
>{
> private static final int NUM_MAX_BYTES = 300000;
>
> public static void main (String[] args)
> {
> int i;
> long t1;
>
> t1 = System.currentTimeMillis();
>
> System.err.print("memory alloc/free 1 - "+NUM_MAX_BYTES+" bytes. ");
> for (i = 0; i < NUM_MAX_BYTES; i++)
> {
> byte[] p;
> p = new byte[i];
> if (i % 1000 == 0)
> {
> // Runtime rt = Runtime.getRuntime();
> // rt.gc();
> System.err.println("current: "+i);
> }
> }
>
> System.err.println("time taken: %d "+ (System.currentTimeMillis()-t1) +
>"ms");
>
> }
>}
>
>
>======= C++ CODE LISTING =========
>
>#include "stdafx.h"
>#include <stdio.h>
>#include <windows.h>
>
>#define NUM_MAX_BYTES 300000
>
>int main(int argc, char* argv[])
>{
> int i;
> DWORD t1;
> t1 = GetTickCount();
>
> BYTE * p;
> printf("memory alloc/free 1 - %d bytes. ", NUM_MAX_BYTES);
> for (i = 0; i < NUM_MAX_BYTES; i++)
> {
> p = new BYTE[i];
> delete p;
> }
> printf("time taken: %d ms\n",GetTickCount()-t1);
>
> return 0;
>}
>
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